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Raman Spectroscopy Raman Spectroscopy ► The Raman effect is based on inelastic scattering of a monochromatic incident radiation - routine energy range: 200 - 4000 cm–1 - comprises a very small fraction, about 1/107 of the incident photons. virtual state Scattered Excitation v” = 1 v” = 0 Infrared Raman (absorption) (scattering) ► Great for many samples! - minimal sample preparation (gas, liquid, solid) - compatible with wet samples and normal ambient - sample fluorescence is problematic!! The history of Raman effect ► Discovered in 1928 by Sir Chandrasekhra Venkata Raman, using sunlight as a source, telescope as a collector, his eyes as a detector. ► Subsequent mercury sources replaced by lasers in 1962 ► Photographic plates replaced by photomultiplier tubes by 1953 ► Double and triple monochromators introduced in 1960s; holographic gratings in 1968 ► FT-Raman, array detectors, Raman microscopes more recent ► Sir Chandrasekhara Venkata Raman was the first scientist to describe and explain in the review Nature, in 1928 March 31, the experimental observation of this phenomenon in liquids. It must be recalled that the theoretical principle of this effect was predicted in 1923 by A. Smekal, an Austrian physicist. Other research groups was working on similar subjects at the same time (during 1928): - L. Mandelstam, G. Landsberg in URSS (May 6) - Jean Cabannes in France (ending of 1928). C. V. Raman and K. S. Krishnan, Nature, 121 (3048), 501, March 31, 1928 If we assume that the X-ray scattering of the 'unmodified' type observed by Prof. Compton corresponds to the normal or average state of the atoms and molecules, while the 'modified' scattering of altered wave-length corresponds to their fluctuations from that state, it would follow that we should expect also in the case of ordinary light two types of scattering, one determined by the normal optical properties of the atoms or molecules, and another representing the effect of their fluctuations from their normal state. It accordingly becomes necessary to test whether this is actually the case. The experiments we have made have confirmed this anticipation, and shown that in every case in which light is scattered by the molecules in dust-free liquids or gases, the diffuse radiation of the ordinary kind, having the same wave- length as the incident beam, is accompanied by a modified scattered radiation of degraded frequency. The new type of light scattering discovered by us naturally requires very powerful illumination for its observation. In our experiments, a beam of sunlight was converged successively by a telescope objective of 18 cm aperture and 230 cm focal length, and by a second lens was placed the scattering material, which is either a liquid (carefully purified by repeated distillation in vacuum) or its dust-free vapor. To detect the presence of a modified scattered radiation, the method of complementary light-filters was used. A blue-violet filter, when coupled with a yellow-green filter and placed in the incident light, completely extinguished the track of the light through the liquid or vapor. The reappearance of the track when the yellow filter is transferred to a place between it and the observer's eye is proof of the existence of a modified scattered radiation. Spectroscopic confirmation is also available. Some sixty different common liquids have been examined in this way, and every one of them showed the effect in greater or less degree. That the effect is a true scattering, and secondly by its polarization, which is in many cases quite strong and comparable with the polarization of the ordinary scattering. The investigation is naturally much more difficult in the case of gases and vapors, owing to the excessive feebleness of the effect. Nevertheless, when the vapor is of sufficient density, for example with ether or amylene, the modified scattering is readily demonstrable. F1 F2 C S “dark” C F1 F2 S “light” Hg lamp C6H6 Raman spectrum 1928 ► Discovered the inelastic Chandrasekhara Venkata Raman (1888-1970) scattering phenomenon 1930 ► The Nobel Prize for Physics Raman scattering: polarizability of molecule changes during vibration! ► Polarizability is related to how easily the molecular orbital of a molecule can be deformed ►An electric field will distort the molecular orbital (electron cloud) ►This is a weak effect that grows with the square of the electric field intensity The electric field can distort the electron cloud of a molecule, creating an “induced” electric dipole moment mind = aE Polarizability (a) The induced electric dipole moment ► The oscillating electric field of the incident electromagnetic radiation will create an oscillating induced electric dipole moment which will emit electromagnetic radiation (scattered radiation) ►Rayleigh scattering: sc ≈ ex elastic interaction; (no transfer of energy between molecule and photon) Stokes ► Raman scattering: ≠ inelastic interaction; sc ex Anti-Stokes (transfer of energy between molecule and photon) Stokes lines: energy of photon decreases: Vibrational Raman selection rules: < (vibrational energy of molecule increases) sc ex Anti-stokes lines: energy of photon increases: ► Polarizability of molecule must change during a particular vibration sc > ex (vibrational energy of molecule decreases) (gross selection rule) ► Δv = ± 1 (vibrational quantum = + or = - AS ex vb S ex vb number/vibrational energy levels) (specific selection rule) Classical theory of Raman scattering (Fourier series) Classical theory correctly predicts that Raman scattering should be weaker than Rayleigh scattering and that there is a simple linear dependence of Raman scattering on incident intensity and on sample concentration. • Expression for induced dipole moment can be written in terms of Cartesian components. • For μ we can use a matrix equation. (μ and E are vectors, α is a tensor) • Polarizability tensor is usually symmetric •Polarizability is divided into isotropic (s) and anisotropic (a) parts: α = αs + αa a’ = a/Q (the polarizability derivative) Boltzmann distribution is a major factor in determining relative Stokes and anti- Stokes intensity. The excited vibrational state will be only thermally populated, and Stokes intensity will be much larger than anti-Stokes. 4 4 h vib hc k I Stokes 0 vib kT 0 k kT e e I antiStokes 0 vib 0 k Rayleigh Stokes Anti-Stokes Raman intensity is proportional with Raman scattering cross-section ( [cm2]) Raman scattering cross-sections = target area presented by a molecule for scattering – depend on excitation frequency - the scattering solid angle 2 -28 2 Process Cross-Section of (cm ) lex (nm) (x10 cm ) absorption UV 10-18 532.0 0.66 -21 absorption IR 10 435.7 1.66 -19 emission Fluorescence 10 368.9 3.76 scattering Rayleigh 10-26 355.0 4.36 scattering Raman 10-29 319.9 7.56 scattering RR 10-24 282.4 13.06 scattering SERRS 10-15 CHCl : C-Cl stretch at 666 cm-1 scattering SERS 10-16 3 Aroca, Surface Enhanced Vibrational Spectroscopy, 2006 Stokes vs Anti-Stokes Rayleight ν = ν Ray ex Ray ex 1 Stokes νStokes = νex- νvib S ex vib ex lex Anti-Stokes νAntiStokes = νex+ νvib AS ex vib CCl4 Stokes: S ex S vib Anti-Stokes: AS ex AS vib The intensity of Stokes bands is not temperature dependent. The intensity of Anti-Stokes bands is temperature dependent. A vibration is Raman active if the polarizability of molecule changes during vibration: a 0 Q k 0 The value of first derivative of polarizability at the origin (tangent, slope) influence the intensity of Raman band. For small molecule, polarizability can be figured like an ellipsoid: Symmetric stretching vibration of CO2 Asymmetric stretching vibration of CO2 ► Polarizability changes during ► Polarizability does not change during vibration vibration → Raman band at 1340 cm-1 → no Raman band near 2350 cm-1 ►Dipole moment does not ► Dipole moment does change → no IR absorption band at 1340 cm-1 → IR absorption band at 2349 cm-1 1 vibration is Raman active (the ellipsoid size is changing). 2 and 3 vibrations are Raman inactive (the variations of size and shape are only apparent!) - in extreme positions the ellipsoid has the same shape and size therefore, the function (da/dQ)0 slope at the origin is zero. ν1 vibration - Raman active (the ellipsoid size is changing). ν2 vibration - Raman active (the ellipsoid shape is changing). ν3 vibration - Raman active (the ellipsoid orientation is changing). The symmetric stretch (ν1) determine a very strong Raman band, while asymetric vibration (ν3) are very weak. In practice is accepted the idea that Raman spectrum of H2O molecule has a single band at 3652 cm-1!!! (due to symmetric stretching) Depolarization ratio The Raman scattered light is emitted by the stimulation of the electric field of the incident light. Therefore, the direction of the vibration of the electric field (polarization direction) of the scattered light might be expected to be the same as that of the incident light. In reality, however, some fraction of the Raman scattered light has a polarization direction that is perpendicular to that of the incident light. This component is called the perpendicular component. The component of the Raman scattered light whose polarization direction is parallel to that of the incident light is called the parallel component, and the Raman scattered light consists of the parallel component and the perpendicular component. The depolarization ratio (in Raman spectroscopy) is the intensity ratio between I the perpendicular component and the parallel ρ component of the Raman scattered light I|| 2 4 I m mx=axyEy my=ayyEy 2 I Ix αxy ρ 2 I|| Iy αyy 3γ2 ρ Rayleigh: 2 45α 4γ2 1 α (α α α ) 3 xx yy zz 1 γ2 [(α α )2 (α α )2 (α α )2 6(α2 α2 α2 ) 2 xx yy yy zz zz xx xy yz xz α is non-zero only for totally symmetric vibrations. For totally symmetric vibrations 0 ≤ ρ ≤ 3/4 For non-totally symmetric vibrations ρ = 3/4 3γ2 Raman: ρ (Polarised laser radiation: ρmax= 3/4) 45α' 4γ2 a α α' ' If laser radiation is unpolarised: Qk Q k ρmax= 6/7.
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